The objectives of this project are to understand the molecular mechanisms utilized by cells for regulating the lipid composition of their membranes, and to establish the processes that lead to the assembly of the lipid bilayer. Two current problems associated with these goals are why the lipid composition of membranes changes when growth temperatures are altered, and why membrane bilayers are unilamellar. Our studies with aqueous phospholipid dispersions indicate that the unilamellar (single bilayer) state forms spontaneously, but only at a unique temperature that depends on the composition of the phospholipid in the dispersion. The temperature at which the unilamellar state forms is a critical point T*; above and below this temperature, other lipid states form which are not unilamellar. The lipid composition requirements for the formation of the unilamellar state have been identified for a number of synthetic phospholipid mixtures, and for a series of cell membrane lipid extracts. In each case the critical temperature T* is characteristic of the lipid composition. Moreover, in the case of the membrane lipid extracts, T* was the same as the growth temperature of the cell from which the lipids were extracted. Since T* represents the temperature where only the single bilayer state forms, the agreement between T* and cell growth temperatures suggests that membrane bilayer assembly in cells may occur by a spontaneous process similar to that observed in the lipid dispersions. This assembly mechanism also accounts for the composition changes encountered when growth temperatures are altered. Since the single bilayer state forms with a characteristic composition at T*, any change in the growth temperature would require a corresponding change in lipid composition if new membrane bilayer is to form. A rigorous test of this hypothesis is currently in progress.